Seven U.S. Marines were killed and at least seven others were injured when a powerful 60mm mortar exploded during a training exercise in Nevada, the Pentagon said today.

The survivors of the blast are being treated at Renown Hospital in Reno for injuries that includes trauma fractures and vascular injuries, hospital spokeswoman Stacy Kendall told ABC News.

Five patients are listed in serious condition, one patient is in fair condition and one patient has been discharged from the hospital, according to Kendall.

The Marines were all from the 2nd Marine Division based at Camp Lejeune, N.C., and were participating in a mortar firing exercise at the Hawthorne Army Depot 140 miles southeast of Reno, Nev. The blast occurred just before 10 p.m. MST on Monday, according to a statement from the Marines.

It takes several Marines to fire a 60mm mortar and they must "work together to provide constant and accurate high-angle suppressive fire," according to the Marines' website.

Several defense officials said this incident did not appear to involve a round landing on the troops.

The mountainous desert terrain of the 230 square mile depot is used as a training location for special forces since it "provides a realistic simulation of the situation in Afghanistan," according to the depot's website. The depot is also used as a storage site for ammunition awaiting demilitarization.

Maj. Gen. Raymond C. Fox said it was important to remember the courage and sacrifice of the Marines.

"We send our prayers and condolences to the families of Marines involved in this tragic incident. We remain focused on ensuring that they are supported through this difficult time," Fox said in a statement.

The Marine Corps is notifying families before releasing the identities of those who were killed. The explosion is under investigation.